The sensor on the roof is for vehicle speed. You see cars with this sensor when BMW is testing different tire sizes.

To reiterate the M2 info we've posted thus far:

Quote:

The 2016 M2 (F87) is taking shape, now testing with the quad exhaust that's become a hallmark of BMW M vehicles. This time the prototype is fitted with wheels from the M3/M4 prototype which did not make production. Earlier prototypes were testing with the M3/M4 19" Style 437M wheels.

As before, we see the aggressively flared fenders and wider track, M brakes (which will be painted blue on the production M2), drilled rotors and M limited slip differential.

Although its front and rear bumpers are now taped up, they remains M sport bumpers (like the earlier prototypes). We will see the M2 bumpers, along with any other cosmetic bits added later in testing.

The M2 will begin production in late Fall 2015, for deliveries in late 2015 - early 2016. It will feature the N55 turbo inline 6 engine (N55B30T0), producing around 370-380 PS (365-375 HP).